Bone remodeling is the dynamic process by which tissue mass and skeletal architecture are maintained. The process is a balance between bone resorption and bone formation, with two cell types thought to be the major players. These cells are the osteoclast and osteoblast. Osteoblasts synthesize and deposit new bone into cavities that are excavated by osteoclasts. The activities of osteoblasts and osteoclasts are regulated by many factors, systemic and local, including growth factors.
One of the growth factors believed to be involved in bone homeostasis is platelet-derived growth factor (PDGF). Biologically active PDGF is found as a homodimer or a heterodimer of the component A and B chains. In vitro studies have shown PDGF to be mitogenic for osteoblasts (Abdennagy et al. Cell Biol. Internat. Rep. 16(3):235-247, 1992). Mitogenic activity as well as chemotactic activities associated with PDGF have been demonstrated when the growth factor is added to normal osteoblast-like cells (Tuskamota et al. Biochem. Biophys. Res. Comm., 175(3):745-747, 1991) and primary osteoblast cultures (Centrella et al. Endocrinol. 125 (1):13-19, 1989.). Recent studies have demonstrated that the osteoblast produces the AA isoform of PDGF (Zhang et al., Am. J. Physiol. 261:c348-354, 1991). The exact mode by which PDGF affects the growth of osteoblasts is not yet clearly understood, however, there does appear to be consensus that the growth factor plays a key role in the regulation of both normal skeletal remodeling and fracture repair.
The therapeutic applications for PDGF include, for example, the treatment of injuries which require the proliferation of osteoblasts to heal, such as fractures. Stimulation of mesenchymal cell proliferation and the synthesis of intramembraneous bone have been indicated as aspects of in fracture repair (Joyce et al. 36th Annual Meeting, Orthopaedic Research Society, Feb. 5-8, 1990. New Orleans, La.).
Vitamin D has traditionally been considered essential for the prevention of rickets, a disease of inadequate bone mineralization. This importance is associated with vitamin D's role in facilitating gastrointestinal uptake of calcium and the importance of serum calcium levels for bone homeostasis. Recent evidence suggests that osteoblasts have receptors for the vitamin D metabolite 1.alpha.,25-dihydroxycholecalciferol, indicating that the osteoblast is a major target for the hormone (Suda et al. J. Cell. Biochem., 49:53-58, 1992). Vitamin D is believed to play an important part in activation by the osteoblast of osteoclast-mediated resorption (Watrous et al., Sem. in Arthritis and Rheum., 19(1).:45-65, 1989). Vitamin D has been used in the in vitro culture of osteoblasts (Kurihara et al., Endocrinol. 118(3):940-947, 1986) and has been associated with an increase in alkaline phosphatase, a marker of cell differentiation into the osteoblastic phenotype. However, in human bone cells alkaline phosphatase stimulation has been associated with a decrease in cell proliferation (Huffer, Lab. Investig. 59(4):418-442, 1988). In calvarial cultures the addition of vitamin D increases the release of calcium into the medium and is correlated to bone resorptive activity (Bell, J. Clinical Investig. 76.:1-6, 1985). Expression of osteocalcin, a marker for osteoblasts, requires vitamin D induction (Yoon et al. Biochem. 27:8521-8526, 1988). The exact role of vitamin D in bone homeostasis and how it exerts its effects on the osteoblast and osteoclast remain to be elucidated.
Because of the important role of osteoblasts in the healing and regeneration processes of bone, an ability to enhance the proliferation of these cells remains a desirable objective. The present invention provides this ability and other advantages as will be apparent from the following detailed description and attached drawings.